1. Field of the Invention
The present invention generally relates to a method and apparatus for sensing small fluid samples in a vessel and, more particularly, to a method and apparatus for determining the volume of a fluid in a vessel, such as a test tube, using a bottom sensing device (e.g., tip jam device).
2. Background Description
Analyses of fluids, especially bodily fluids such as urine and blood, are important to the diagnoses and treatment of various illness and other conditions. These illnesses and conditions can range from various forms of cancers to blood diseases to drug use and others.
In order to analyze a bodily fluid, a sample of fluid is first taken from a person and analyzed either by hand or by an automatic analyzer or other device of the type well known in the art. In the case of an automatic analyzer, for example, the bodily fluid is disposed in a tube which, in turn, is disposed on a carousel or other conveying mechanism. The carousel or other conveying mechanism conveys the tube though scanning stations, for example, and under a pipetting station in order for a pipetting probe to aspirate a sample of the fluid.
The pipetting probe is then lowered into the tube in order to aspirate a sample of the fluid. Thereafter, and depending on the specific test or tests to be performed on the fluid, a specific reagent may be combined with the fluid in order for a chemical reaction to occur. This chemical reaction is then analyzed to determine, for example, the amount of analyte in a sample of fluid.
It is not uncommon for many different tests to be performed on the sample fluid using different reagents. However, in order for the appropriate tests to be performed on the sample fluid a sufficient amount of the sample fluid must be present in the tube. Accordingly, when using automatic analyzers, the sample level in the tube is normally determined by the pipette probe which is connected to a sample sensing means such as a capacitive or conductive circuit. The sensing means is triggered upon contact of the pipette tip with the surface of the sample. The pipette probe is then further lowered a distance into the sample sufficient to allow withdrawal of the required volume. However, to ensure that an insufficient volume of sample will not be drawn, owing to the sample level being too close to the bottom of the tube, the pipette tip will only be allowed to be lowered to a certain level within the tube resulting in a volume of sample, called the dead volume, that is unavailable for testing. The maximal distance the pipette tip is allowed to be lowered, and thus the nominal dead volume, is set by the manufacturer of the automatic analyzer. It is noted that the actual dead volume is variable and is dependent on several dimensional tolerances that exist within and between different instruments. A more dimensionally precise automatic analyzer would allow the pipette tip to aspirate fluids at a lower level than other less precise automatic analyzers, and thereby allow the manufacturer to set a smaller sample dead volume. Any amount of sample fluid below the preset dead volume level can not be utilized to perform a test or tests thereon despite the fact that the fluid in the test tube below the dead volume level may still be sufficient to perform a test or tests thereon.
Thus, what is needed is a system that determines the exact volume of a fluid below a threshold level which may be defined as a near bottom tube level. It is noted that the near bottom tube level is an arbitrary level of fluid in the tube, and may be predefined by the manufacturer of an automatic analyzer. The determination of the exact volume of a fluid in the tube will allow the automatic analyzer or other device to determine whether there is a sufficient amount of fluid in the tube in order to perform a certain predetermined test or tests.
It is therefore an object of the present invention to provide a system and method for determining a bottom of a tube or other container.
It is a further object of the present invention to provide a system and method for determining the volume of a fluid in a tube or other container.
It is also an object of the present invention to provide a system and method of measuring a reagent in a reagent container.
It is still another object of the present invention to provide a system and method for determining whether a sufficient amount of fluid is present in a tube for a specific test or tests.
In order to accomplish the objects of the present invention, a pipetting station having a bottom sensing device is provided in conjunction with one of any known liquid level sensing devices. The bottom sensing device includes a pipetting probe spring mounted to a pipetting arm of the pipetting station. The bottom sensing device also includes a sensor for determining when a pipetting tip of the pipetting probe is in contact with a bottom of a tube.
The bottom sensing device of the present invention permits the pipetting probe to measure the volume of fluid in the tube by allowing the pipetting tip to be lowered to the bottom of the tube beyond the sensed fluid level (and the near bottom tube level). In the embodiments of the present invention, the pipetting arm is further lowered until the pipetting probe triggers a sensor which stops the downward movement of the pipetting arm. The exact distance between actual tip jam and triggering of the tip jam sensor, and therefore the actual bottom of the tube, is known and configured for each instrument.
A determination is then made as to (i) an exact volume of fluid and (ii) whether there is sufficient sample fluid in the tube to perform a test or tests thereon. The determination of the volume of fluid in the tube is based on (i) the sensed level of the fluid as determined by the level sensor (in relation to a xe2x80x9chomexe2x80x9d position of the pipetting arm), (ii) the distance the pipetting arm traveled from the level of the sample fluid to the time when the pipette tip contacts the bottom of the tube and (iii) the type of tube used for holding the fluid. If sufficient fluid is present, then the pipetting tip is raised slightly above the known tube bottom level in order to aspirate sample fluid or reagent therefrom.